Adult narcissists can rarely be "cured", though some scholars think otherwise. Still, the earlier the therapeutic intervention, the better the prognosis. A correct diagnosis and a proper mix of treatment modalities in early adolescence guarantees success without relapse in anywhere between one third and one half the cases. Additionally, ageing moderates or even vanquishes some antisocial behaviours.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 2 DVDs with 12 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)